This invention relates to a moulded structural tub for a vehicle. In particular the invention relates to a fibre reinforced composite tub having a shape selected so as to be suitable for resin transfer moulding.
Fibre reinforced composite materials (or “composites”) such as carbon fibre reinforced polymers (CFRP) are being increasingly used in automobiles, particularly sports cars, because such materials can provide a high strength to weight ratio. Replacing conventional metal structures with composite materials allows the weight of an automobile to be reduced, which improves performance and reduces emissions. Because of their strength, composites are particularly suitable for use in the structural components of an automobile. However, the use of such composites are limited by their expensive. The material parts of the composite are themselves expensive (carbon fibre is especially expensive) but, particularly for larger components, a significant part of the cost lies in their manufacture.
Less complex, smaller composite components can be made using moulding techniques such as resin transfer moulding (RTM), which can be readily scaled up for mass production. Body panels, vehicle trim and strengthening beams can often be made in this way. But more complex and larger components are conventionally laid-up by hand using prepreg structures (arrangements of composite fibres pre-impregnated with the matrix material used to bond the fibres together) and cured in an autoclave. This can be a very time-consuming process, and preparing and laying-up the prepreg structures requires expensive skilled labour.
One particular example of a complex composite structure which is currently manufactured using a prepreg and autoclave technique is the composite tub used as the basic structural platform in some racing cars. Such tubs are large parts and require a complex structure in order to give them sufficient strength and stiffness. These composite tubs are now finding their way into high-performance sports cars which can benefit from the excellent strength and stiffness provided by a composite tub. However, because of the expense of manufacturing a composite tub using a prepreg and autoclave technique, the use of composite tubs is limited to very expensive “supercars”.
The large and complex structures of a tub are manufactured using a prepreg and autoclave technique because any moulding tool capable of making a conventional tub would itself have to be large and complex, having many tool parts which can operate together to provide a suitable mould for the tub but which also allow the tub to be released from the mould once it is complete. Aside from the difficulties inherent in making large and complex structures using composite moulding techniques such as RTM, this complexity means the tool would be very expensive. Since tubs are often only manufactured in small numbers (e.g. in the case of racing cars and supercars), the cost of the moulding tool would often outweigh the cost benefit of using faster and less labour-intensive moulding techniques.
Complex structures can be made from moulded composite materials by gluing together more straightforward pieces which can be made using moulding techniques, but the bonds between the pieces form weak spots in the structure at which failure is more likely to occur. It should also be noted that the complexities of a structure generally lie in the features which give the structure strength, such as box sections, corrugations and edges.
It would be desirable if composite structural tubs could be manufactured using less expensive and less labour-intensive techniques which allow the rapid manufacture of tubs for vehicles. This would mean automobile manufactures could use composite tubs in cars assembled according to high volume processes, allowing their use in less expensive cars.
There is therefore a need for a composite single-piece tub structure suitable to be manufactured using moulding techniques, without compromising the strength of the tub design. There is also a need for a composite single-piece tub structure that can be made by such moulding techniques in high volumes using a moulding tool having a minimal number of tool parts.